The hetero-junction bipolar transistor (HBT) is known as a high frequency transistor which can supply a high current. On the other hand, since GaN-based semiconductor is a widegap semiconductor, it is possible to apply a high voltage. Therefore, if a hetero-junction bipolar transistor is made by GaN-based semiconductor, it is expected that a high frequency power transistor having high resistance voltage and high power is realized.
Until now, the following GaN-based hetero-junction bipolar transistor is known (see patent literature 1.). The GaN-based hetero-junction bipolar transistor is made as follows. An AlN buffer layer, a GaN buffer layer, an emitter layer made of n-type AlGaN, a base layer made of p-type GaN, a collector layer made of n-type AlGaN and a subcollector layer made of n-type GaN are grown in order. Then the collector layer and the subcollector layer are patterned into a prescribed shape by etching and further the base layer is patterned into a prescribed shape by etching. Finally, an emitter electrode is formed on a part of the emitter layer where the base layer is removed, a base electrode is formed on a part of the base layer where the collector layer and the subcollector layer are removed and a collector electrode is formed on the subcollector.